Electrolytic cells have many and diverse commercial applications. These include the electrochemical manufacturing of chlorine, sodium, and fluorine, and the electrolytic generation of oxygen and hydrogen.
An electrolytic cell typically includes a vessel within which are enclosed positive and negatively charged electrodes, and an electrolyte that provides ionic contact between the anode and cathode electrodes. Where mixing of the products or reactants of electrolysis is undesirable, the cell is built with a diaphragm to separate the anode and cathode compartments. For example, in the electrolysis of brine to produce chlorine gas and hydrogen, any substantial mixing of the products is extremely hazardous, posing a great risk of explosion. Similarly, the hydrogen and oxygen generated from the electrolysis of water in an alkaline aqueous electrolyte, e.g., 30% KOH, must be effectively separated to avoid the risk of explosion. A porous diaphragm that permits ionic and hydraulic flow between the anode and cathode elements while preventing the diffusion of gasses is typically employed to separate the chlorine and hydrogen of brine electrolysis or the products of oxygen generation. It is desirable that the cross-cell voltage differential across the diaphragm be as small as possible to allow efficient operation of the cell. The qualities that make a diaphragm material desirable in an electrolytic cell are also important in electrochemical cells, such as batteries and fuel cells.
It is often desirable to operate electrolytic cells at pressures in excess of atmospheric pressure. For example, in an oxygen generating cell, a 200 fold increase in pressure results in an increase in reversible potential of only about 103 mV. The theoretical reversible cell voltage varies with pressure in this manner: ##EQU1## Therefore, high pressure gas can be generated with only a minimal increase in electrolysis voltage. Electrolytic oxygen generators of the filter-press type are commonly constructed to operate at pressures of 30 atmospheres, eliminating the need for compressors in the gas line.
As a function of both space constraints and the large output required, oxygen generators for use aboard submarines operate at even higher pressures, typically about 200 atmospheres. In addition, submarine oxygen generators operate at elevated temperatures, often in excess of 300.degree. F. An additional concern in submarine oxygen generators, for obvious reasons, is reliability and safety. The diaphragm is a critical component of such high pressure oxygen generators.
As discussed above, a diaphragm for use with an aqueous electrolyte must permit ionic and hydraulic flow, but effectively separate any gasses generated in the operation of the electrolytic cell. Impermeability to gasses becomes increasingly critical as operating pressure and temperature is increased. At the same time, however, the diaphragm must remain wettable and maintain a low cross-cell voltage differential. In addition, it is desirable that the diaphragm remain stable over prolonged periods of exposure to a highly caustic electrolyte at elevated temperatures.
Diaphragms for electrolytic cells are commonly made of asbestos. Asbestos diaphragms have the desirable properties of resistance to degradation when exposed to a caustic electrolyte at low and moderate temperatures, low gas diffusion rates under high pressure operating conditions, good ion transfer capabilities with low ohmic resistance, adequate osmotic and hydraulic flow rates, and sufficient tensile strength for durability and convenience both in handling and in cell construction. However, it is desirable to find a substitute material to replace asbestos diaphragms for electrolytic cells. Asbestos poses health risks to workers who are exposed to it in the dry form, and asbestos deteriorates over time under the severe operating conditions commonly found in electrolytic cells. Also, under high temperature conditions asbestos reacts chemically with concentrated caustic to form soluble silicates and magnesium hydroxide (Brucite). This limits the useful life span of asbestos in many applications. Synthetic polymers, on the other hand, like polyphenylene sulfide and polyetheretherketone, do not react chemically with caustic and therefore last longer under the same operating conditions. This is an improvement over the present state of the art.
Asbestos is a unique diaphragm material, being a hydrophilic, durable, mineral fiber that can be woven into a fabric. Any substitute material for asbestos would desirably match or exceed asbestos with respect to its desirable qualities, and not suffer from the above-mentioned disadvantages. Natural textiles generally lack the necessary stability when exposed to caustic electrolytes. Synthetic fibers are generally hydrophobic with insufficient wettability for diaphragm applications.
Fabrics are characterized generally as woven or unwoven. Woven fabrics are made from yarn. Warp yarns are stretched across a loom at considerable tension, and filling yarns, i.e., weft or woof yarns, are inserted and interlaced at right angles to the warp. Warp yarns are generally more highly twisted and less extensible than filling yarns.
Yarns are generally of two types: multifil and staple. A multifil yarn is made from individual very long continuous fibers. A staple yarn is made from shorter fibers, typically 1/2 to 3 inches each in length, which are spun to form the yarn. In general, yarns formed from staple fibers are bulkier, softer, and have a rougher surface texture than continuous multifil yarns. Beyond a certain minimum length required for successful manufacturing of staple yarn, the length of the individual fibers has very little effect on the properties of the resulting yarn. In contrast, the denier, a measure of linear density, of the individual staple fibers has a striking effect on the resulting yarn. Filament fibers that make up monofil yarn are, for the most part, synthetics. Asbestos is a staple fiber. Silk is the only commonly used natural textile material where the individual fibers are long enough to make multifil yarn. Synthetic filaments are formed by extrusion processes e.g., melt, dry, or wet spinning. Common synthetics include polyacrylics, rayon, polyester, nylon, polyethylene and polypropylene. Long continuous synthetic filaments can be processed directly into multifil yarns. Short staple fibers can be carded and spun into staple yarn.
Felt is made by pressing staple fibers together and can be accomplished through the action of heat, moisture, chemicals, and pressure. This process may also include the step of needling individual staple fibers into a loosely woven cloth, called a scrim.
In light of the foregoing, it is an object of this invention to provide an asbestos-free diaphragm for use in an electrolytic cell.
It is a further object of the invention to provide a diaphragm that can withstand exposure to a caustic electrolyte at elevated operating temperatures and pressures for prolonged periods of time.
It is a further object of the invention to provide a diaphragm capable of preventing undesirable gas diffusion without undesirably impeding ionic or hydraulic flow rates under elevated pressure conditions.
It is a further object of the invention to provide a diaphragm capable of maintaining a desirably low cross-cell voltage differential when operated under high current density conditions.
It is yet a further object of the invention to provide an asbestos-free electrolytic gas generator capable of operating under severe conditions of temperature and pressure.
It is yet a further object of the invention to provide a process for electrolytically generating oxygen in an electrolytic cell employing an asbestos-free diaphragm.